This patent document relates to optical waveguides.
Optical waveguides are structures that confine and guide light in one or more optical modes of the optical waveguide. Many optical waveguides are structured to include a continuous waveguide core and a waveguide cladding in contact with the waveguide core with a refractive index less than the waveguide core to guide light. One example of such an optical waveguide is a high-index waveguide core layer formed between two waveguide cladding layers each having a refractive index less than that of the waveguide core. Another example is an elongated waveguide core surrounded or clad by a lower-index waveguide cladding such as an optical fiber.
An optical slot waveguide has a different structure than the above core-cladding structure in that it includes a slot formed in the core with a refractive index less than the core. For example, one example of an optical slot waveguide can include a nanoscale slot layer with a refractive index of nS sandwiched between two high-index core layers with a refractive index of nH greater than nS and two low-index cladding layers each with a refractive index of nC less than nH outside the two high-index core layers, respectively. Such a slot waveguide structure provides a discontinuity of the electric field (e.g., for the transverse magnetic mode or TM mode) at high-index-contrast interface to strongly enhance and confine light in a nanoscale region of the slot layer and can be applied to a variety of optical devices such as microring resonators, directional couplers, optical modulators, optical sensors, light enhancement, all-optical logic gates, and multimode interference waveguides. The emergence of these and other applications for slot waveguides leads to integration of a slot waveguide with an optical waveguide that is not a slot waveguide and may be made of, e.g., a high-index core surrounded by low-index claddings for low loss optical interconnections in single photonic chips and other applications. The coupling structures that can efficiently couple light between a slot waveguide and a non-slot waveguide. Due to the difference in size between the cross section of a slot layer in a slot waveguide and the cross section of another optical waveguide not in the slot waveguide configuration, various coupling designs for slot waveguides with other optical waveguides use a tapered waveguide structure with a sufficient length (e.g., several microns or longer) to provide an adiabatic mode transformation between the mode of the slot waveguide and the mode of the other non-slot waveguide to achieve a high coupling efficiency, e.g., over 99%.